Untitled Document

Physics Demo Number: 078

Approximate Run Time: 10 min

Transverse Wave and Pulse Demonstrator

Demo Description

Use a torsional rod apparatus to show the movement of individual points in a transverse wave.

Scientific Principles

Transverse wave motion
Traveling transverse pulses
Standing transverse waves

Equipment

Torsional rod apparatus ( a set of rod masses rigidly attached to a stiff piece of torsion-wire)
Protective Case for Apparatus
Black Light (optional)

Equipment Location

The case lives hanging on the wall at end of [E]-[F] Aisle.
The Torsional Rod Apparatus Lives in the Case.
A Black-Light Lives on [C-2-5].

Instructions

The photo shows the torsional rod apparatus for demonstrating transverse waves and pulses, resting on its protective wooden storage case.

One may illuminate the greenish rod tips with a Black-Light to cause them to fluoresce for added visual effect.

One simply grasps the very end rod (or two rods at one end) between the thumb and forefinger of one hand at the center of the rod(s) ( with the thumb on the top side of the rod(s) and the forefinger on the bottom of the rod(s)) and executes a crisp 45 to 90 degree twisting motion of the wrist (followed by a crisp reverse twist of the wrist) of that hand to send a single well-defined pulse traveling along the rod-array.

The pulse will reflect at the opposite end and come back to the originating end of the array.

A little practice in driving the end rods with appropriately timed and oriented wrist twists will allow one to set up a standing wave in the rod array.

With the room lights darkened and the black light on the greenish rod tips , it will be easy for the students to see ( thanks to the effect of persistence of vision) that all the rods in the loop between two successive nodes of the standing wave oscillate in phase with each other and 180 degrees out of phase with the rods in the two loops to each side of their loop--- once you point this behavior out to them.

Moreover, one may point out that in the loops of rods oscillating in phase with each other ,each rod is evidently undergoing simple harmonic motion, since the overall visual display appears to be an accurate sine wave.